Conventionally, various techniques are known for the communication between a terminal and a base station for networking those terminals and stations. For example, cellular phone services provide voice call services and packet data communication services through cellular phone networks for cellular phone terminals. In this case, for notifying the user about the availability of the call service at the moment, the cellular phone displays an in-area icon and an out of service area icon based on the signal intensity or Ec/Io (i.e., a ratio of received pilot energy (Ec) to total received energy or the total power spectral density (Io)). For example, a Japanese patent document JP-A-H10-108261 discloses the determination method of the in-area and out of service area of the service based on the received electrical field intensity and errors in the modulated digital signal that is derived from the received radio wave.
Further, in the telematics services, map data and POI (i.e., Point Of Interest) data are downloaded and diagnosis information is uploaded from and to a data center by a vehicular communication module through the cellular phone network, for example. Furthermore, if the voice call service for voice call to a service center is not desired, only packet data communication service can be purchased at a lower service rate for the data upload and download.
The network operators of the cellular phone networks basically build their own networks by themselves, by providing base stations for themselves, for the purpose of accommodating subscribed users by allowing them to access their own base stations for the mobile communication services. However, it is impossible for each of the network operators to cover all of the service areas only by their own base stations. Therefore, the network operators share their base stations with other operators, through roaming contracts, for the purpose of providing the subscribed users with the extended service through the other operator's networks and stations for the increased coverage.
In that case, even when roaming of the voice call is guaranteed through base stations and networks of different operators, roaming of the packet data communication may not necessarily be guaranteed for the roaming user.
For example, in the cellular phone services, the access to a certain base station is determined based on the signal intensity and/or Ec/Io measurement results, as described above, with the display of the in-area/out of service area icons. That is, the in-area/out of service area icons on the cellular phone are displayed for notifying the user of the availability of the voice call only, because the telephone network has primarily been built up for the call switching of the voice call.
However, the packet data communication through the cellular phone network is an extension of the voice call system to a data communication system, thereby suffering from various limitations of the voice call system. That is, for example, even when the signal intensity is strong enough, the packet data communication may not be available for various reasons such as: (a) the base station does not support the packet data communication function, (b) the packet data communication function in a specific base station is temporarily not working due to some failure, (c) the roaming partner's network does not support the packet data communication function, (d) the roaming partner's network does not support roaming of the packet data communication function, and the like.
Therefore, the in-area/out of service area icons on the cellular phone do not correctly notify the user of the availability of the packet data communication from the “available” base stations in the connectable range, thereby causing the inconveniences for the user of the packet data communication.